A motor cooled by a coolant can generate about 1.5 to 2 times as much power as a motor cooled by air, although the two motors have the same size, since the coolant has a high cooling ability. Furthermore, the coolant-motor can reduce an increase of a temperature of each part of the motor, due to a heat generated by a continuous high load, a core loss during a high revolutional speed, frictional loss of a bearing, and so on. Moreover, a channel for a coolant must be formed in a motor, and accordingly, in the prior art, a channel is formed when a casing of a motor is cast, and then a portion to be covered is covered by a plate member.
Generally, a motor is attached to a machine by the front end of the casing thereof, and accordingly, a coolant must be supplied from the rear of the motor, returned to the rear again, and after this cycle is repeated several times, the coolant must be returned to a tank from the rear. Therefore, a communicational channel must be formed when casting the casing so that those longitudinal channels are communicated with one another, and accordingly, the casting of the casing is very difficult when producing a casing with such a defect.